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Life Cycle Engineering of Carbon Fibres for Lightweight Structures
Available online at www.sciencedirect.com
ScienceDirect Procedia CIRP 69 (2018) 43 – 48
25th CIRP Life Cycle Engineering (LCE) Conference, 30 April ± 2 May 2018, Copenhagen, Denmark
Life cycle engineering of carbon fibres for lightweight structures Antal Déra,d,*, Alexander Kaluzaa,d, Denis Kurlea, Christoph Herrmanna,d, Sami Karab, Russell Varleyc a
Chair of Sustainable Manufacturing and Life Cycle Engineering, Insitute of Machine Tools and Production Technology (IWF), Technische Universität Braunschweig, Langer Kamp 19b, 38106 Braunschweig, Germany b Sustainable Manufacturing & Life Cycle Engineering Research Group, School of Mechanical & Manufacturing Engineering, The University of New South Wales, Sydney, NSW 2052, Australia c Carbon Nexus, Institute for Frontier Materials, Deakin University, Geelong, Victoria 3216, Australia d Open Hybrid LabFactory e.V., Hermann-Münch-Straße 2, 38440 Wolfsburg, Germany * Corresponding author. Tel.: +49-531-391-65035; fax: +49-531-391-5842. E-mail address: [email protected]
Abstract Composite materials are of major importance in today´s manufacturing of lightweight structures. Due to their excellent material properties, carbon fibre reinforced plastics (CFRP) have become highly desired engineering materials for structural applications. However, energy-intensive thermal processes as well as long cycle times during manufacturing have been an obstacle towards high-volume applications. This paper presents a framework for assessing the eco-efficiency of carbon fibre production. The developed framework enables interdisciplinary collaboration between different engineering disciplines by combining methods and tools to ensure life cycle improvements in carbon fibre production. ©201 2017The The Authors. Published by Elsevier B.V. © Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license Peer-review under responsibility of the scientific committee of the 25th CIRP Life Cycle Engineering (LCE) Conference. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 25th CIRP Life Cycle Engineering (LCE) Conference
Keywords: Carbon fibre lightweighting; eco-efficiency; interdisciplinary engineering; life cycle engineering
1. Introduction Maintaining our standard of living greatly relies on transportation and industrial production that requires large amounts of energy and releases greenhouse gas emissions (GHG-emissions) into the atmosphere. Over one third of global man-made GHG emissions is generated by industry from making goods while another quarter comes from the use of transport [1, 2]. Besides alternative drivetrains like in electric mobility, the application of lightweight structures is a promising approach towards reducing the energy demand of vehicles and thus more sustainable mobility [3]. The application of fibre-reinforced plastics (FRP) plays a major role in the design and manufacturing of lightweight structures. FRP show specific advantages encompassing high strength and stiffness, low density, ease of shaping as well as a high durability in relation to other materials [4]. In high load carrying structural applications, carbon fibre-reinforced plastics
are preferable engineering materials (CFRP), both, with thermoset and thermoplastic matrix material. In automotive applications, weight reductions of up to 60 % can be realized in comparison to conventional steel designs [5]. Manufacturing of carbon fibres with state-of-the art process technology leads to high energy intensities in comparison to other lightweight engineering materials. Depending on the share of fibres in the composite (typically between 30 and 50 % by volume) including the matrix and fibre type, a cumulative energy demand that exceeds engineering steel by a factor between 5 and 10 per kilogram of material is reported [6]. Consequently, this influences the cost structure of components manufactured from CFRP, hindering the application of CFRP in price-sensitive industries [7, 8]. Towards an application in competitive product designs, burdens from manufacturing need to be compensated by benefits gained from other stages of the life cycle. Figure 1 depicts the impact of using lightweight CFRP components from an environmental perspective within
2212-8271 © 201 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 25th CIRP Life Cycle Engineering (LCE) Conference doi:10.1016/j.procir.2017.11.007
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the automotive life cycle. Lightweight automobiles have a lower energy demand and associated emissions in the use phase compared to conventionally built heavier vehicles. However, the energy intensive production of lightweight materials leads to a shift of the lifecycle hotspot towards upstream processes [9]. Consequently, the break-even point between the environmental impact and the driven distance moves further out on the mileage or time line, eventually exceeding the life of an automobile. While being an alternative to conventional materials in industries with long product life cycles and thus sufficient pay off timescales, e.g., aerospace applications, CFRP materials are currently not competitive in large-scale automotive production. To ensure the environmental advantages of lightweight structures, this break-even point has to be reached earlier in the use phase. An approach (highlighted in Figure 1) leading to this vision is to reduce the initial environmental impact during the raw material extraction and SURGXFWLRQSKDVHRIFRPSRQHQWVE\WKHIDFWRUĮ7KLVUHVXOWVLQ a shift oIĮ¶GXULQJWKHXVHphase that enables the break-even point to be reached sooner. Increasing the eco-efficiency of carbon fibre production could initiate the shift towards an earlier break-even point and be realised by reducing the energy intensity and the associated environmental impact of carbon fibre production. Eco-efficiency describes within this context the ratio between the value creation of production and its environmental impact [10]. Against this background, this paper aims to introduce a framework for assessing the eco-efficiency of carbon fibre production by combining methods and tools from different engineering disciplines.
Fig. 1. Environmental impact of using CFRP components over the automotive life cycle
2. Challenges of carbon fibre production from a LCE perspective Staying with the example of the automotive industry, the increased environmental impact in the production phase of CFRP is difficult to be compensated for within a reasonable mileage [9, 11]. In vehicle applications, the break-even point of lightweight materials is as well strongly dependent on the applied drivetrains. This is highlighted by the example of electrification. Figure 2 presents different lightweight options based on material substitutions in comparison to conventional
steel designs. It relates greenhouse gas emissions originating from the manufacturing with those resulting from the electricity mix applied for charging. The y-axis then represents the required mileage towards achieving an environmental break-even point based on state-of-the-art technologies [12]. It is to be observed that currently only markets with high shares of fossil energies seem to be advantageous for achieving the break-even point for the CFRP-lightweighting strategy of electric vehicles.
Fig. 2. Break-even of lightweight material substitutions for electric vehicles depending on GHG emissions from electricity mix applied for charging [12]
Carbon fibre-reinforced composite structures pose further challenges to the end-of-life phase of products, especially in closing the loop through re-manufacturing the recyclates into the secondary market [13]. With regard to electric vehicles the interdependencies between lightweight of the car, the battery size and the driving range are important. Reducing the manufacturing costs and environmental impact of carbon fibre production and solving the environmental obstacles in the end of life phase will undoubtedly increase demand from the automotive sector wanting to take advantage of the many advantageous properties of composite materials [7, 8, 14]. To do this, it is paramount that carbon fibre producers gain a comprehensive understanding of the interdependencies between the main production processes and quality related properties. This further includes the design and stable operation of carbon fibre production lines to improve the energy intensity and reduce the associated costs of the production processes. A first step towards improving the carbon fibre technology from a life cycle perspective is understanding its production and the underlying interdependencies between the system elements. Therefore, the next sections lead through the steps of carbon fibre production. 2.1. The carbon fibre production process Carbon fibre is a fibrous material containing at least 92 weight percent carbon [15]. It is formed in a series of processing steps from a precursor to a high-strength and highmodulus fibre. Today´s precursor materials are almost exclusively based on polyacrylonitrile (PAN) but alternatives like pitch and rayon can be employed [15]. As shown in Figure 1, carbon fibre production can be broken down into four main steps: (1) precursor fabrication, (2) stabilization (also referred to as oxidation), (3) carbonization and (4) surface treatment. Although the sequence of the processing steps
Antal Dér et al. / Procedia CIRP 69 (2018) 43 – 48
remain the same regardless of the precursor type, the process conditions have to be adapted to the precursors individually. Precursor fabrication determines the material properties and final quality of the carbon fibre to a great extent. Moreover, it represents a major cost factor during production and has therefore been intensively investigated in recent years [7]. The following stabilisation and carbonisation steps aim at maximizing the carbon yield from the fibre. To achieve the required carbon content in the fibre, various chemical compounds (for example HCN, NH3 and CH4) are removed under carefully controlled thermal atmosphere [16]. The ongoing removal of non-carbon compounds results in a corresponding weight loss and shrinkage in fibre diameter as well as in a gradual change in fibre colour towards black. While stabilisation takes place between 200-300°C and stabilisation time is measured in hours, carbonisation requires only a few minutes at temperatures up to 3000°C [15]. The final process step, surface treatment conduces to better post-processing qualities of the carbon fibre. For example, increasing the ability to form a bond with the polymer matrix is a desired property in CFRP manufacturing [15]. In order to achieve the economic feasible and environmental benign application of carbon fibre, several challenges and unresolved obstacles need to be understood and eventually overcome. Those challenges are explained in the following bipartite structure; first addressing system relevant challenges and secondly, product, fibre and process associated interdependencies. 2.2. General challenges and drivers related to carbon fibre factories & production lines Challenges and drivers for improvements may vary among producers. While established producers will have to find ways of integrating new carbon fibre technologies into existing production lines, new market participants are more able to take advantage of future carbon fibre technologies during their own start up. Unknown future demands in combination with an increased variety of fibre types further exacerbate planning activities of production lines. Thus, it is of great interest to know the effects and impacts of a scaled-up production, particularly for the predicted growth in the automotive sector. Increased international competition among the few carbon fibre producers leads to additional cost pressures and risks for new market participants, associated with high investments in new and in existing factories. Besides economic pressures, the technological evolution and high environmental impacts of the production process itself, poses further challenges and drivers for the carbon fibre producers. As a result, addressing questions of cost and environmental impact during scale up for simultaneously improving fibre performance and quality becomes an increasing problem. This again leads to more specific questions on how to improve the manufacturing of carbon fibre (e.g., in terms of lead-time, overall equipment effectiveness, yield, scrap, personnel costs) as well as environmental perspectives (e.g., energy demand, emissions and material efficiency). Moreover, addressing individual improvement goals will inevitably affect other targets, such as
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the interconnected aforementioned indicators that create a complex production system. 2.3. Specific challenges and drivers related to products, fibres and processes Besides potentially conflicting goals and interactions between production line equipment and technical building services (TBS), the interaction between the individual production process steps and the properties of the fibres is of particular interest. Nowadays, carbon fibre production is strongly concentrated to a few producers. Furthermore, carbon fibre production is marked by either the experience of the operating personnel and/or various empirical test runs for a specific production line or individual process step [16, 17]. This circumstance impedes the production of not only reliable but also repeatable carbon fibre (e.g., of specific fibre properties) and prevents the ability to precisely control properties through an understanding of the chemistry. Consequently, it remains unknown which process parameters are necessary to achieve certain product characteristics (e.g., defined by product designers). This missing gap in knowledge creates uncertainty in the reliability and quality of the final product and decelerates scale up. In addition, currently there only exists a limited transferability of production line concepts and applications as well as a significant lack of understanding concerning the controllability of resulting fibre and therefore product properties. To meet future demands, there is a strong need for methods and tools supporting an interdisciplinary systems understanding which fosters innovations (e.g., in form of adjustable fibre properties) and an integrated evaluation of improvement measures regarding technical, economic and environmental goals. So far, only specific or isolated approaches have been developed addressing mainly the technological evaluation of either the final carbon fibres or individual process steps. Recent activities include the empirical analysis and development of prediction models for fibre properties and the oxidation process [18, 19, 20]. However, this only represents a snapshot character of the current status of the production line or specific processes, since it neglects to take further process and system interactions into account [21, 22]. The evaluation of the economic and environmental effects of carbon fibre production chiefly relies on rough estimations and assumptions (e.g., assuming an operation at full capacity, which is rarely the case) which have been addressed in several cost and life cycle assessment (LCA) studies [11, 21, 23]. However, those studies are based on static calculations and do not focus on dynamic effects, e.g., on varying process conditions required to produce different carbon fibre qualities for different application areas. A typical method to model and reflect dynamic behaviour is simulation. In manufacturing, diverse simulation approaches exist, which range from process chain over separate machines to entire factories modelling [22]. Yet, only limited effort has been made to develop simulation approaches combining multiple hierarchical scales within a carbon fibre factory and/or link them to product properties.
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Fig. 3. Conceptual framework for assessing carbon fibre production
3. Framework for assessing the eco-efficiency of carbon fibre production To tackle the outlined challenges associated with producing carbon fibres; this paper proposes a new conceptual framework to evaluate carbon fibre production. It involves empirically derived data and analytical relations to foster the interdisciplinary collaboration of relevant research fields such as material sciences, production system simulation and life cycle engineering. Therefore, the proposed concept and its methods can be perceived as a platform for experts from different research areas to share and mutually gain process knowledge and insights into the production of carbon fibres and its dynamics. Figure 3 graphically shows the abstracted and generalized framework for a carbon fibre production. The framework integrates several involved system elements and topics such as carbon fibres and their quality, individual production processes, production equipment and the involvement of TBS as well as diverse interdependencies between all those aspects. It is a key objective of the framework to suggest suitable types of modelling to examine the effects of those interdependencies for individual production line steps, such as processes (e.g., oxidation or low temperature (LT) and high temperature (HT) carbonisation furnaces) as well as changes or propagations of fibre properties along the production line. In order to examine those aspects properly, the framework is further subdivided into different research areas (A-D). Those areas differ concerning their system level ranging from single processes (A) over process fibre interactions (B) to entire production lines (C), also including an application and evaluation (D), which are described in more detail in the following. Area A has the objective to identify and quantify essential relations between carbon fibre properties pursuant to specific production process and system characteristics (e.g., temperature levels in LT and HT furnaces, overall line speed and dwell time). This lays the foundation to adjust relevant process parameters and variables in order to reach defined carbon fibre properties. To achieve that, a bipartite approach is suggested. At first, all established and acknowledged functional and analytical relations between process parameters and carbon fibre properties need to be gathered and selectively
verified. Secondly, different design of experiments have to be conducted to acquire experimental data and subsequently derive empirical knowledge about previously unknown functional relations between process parameters and carbon fibre properties. For understanding the nature of the underlying relations, for example the one factor at a time (OFAT) and the response surface methodology, based on a second-order composite experimental design, are expected to yield good results. The OFAT method varies only one parameter successively while all other parameters are kept constant. Therefore, the OFAT method represents a straightforward and simple realization. However, the OFAT method can be quite time-consuming when interactions between multiple parameters are of interest. For this reason, second-order composite experimental designs might be used to gain knowledge of parameter interaction while providing reasonable robustness against outliers [24]. If necessary, further methods of design of experiment are applicable, as well [25]. The resulting data and functional relations between process parameters and carbon fibre properties are then used as an input for the development of process specific models. Area B bases on the acquired data and functional cause-andeffect relations to model the critical production processes such as the oxidation, LT and HT furnaces to gain a fundamental and transferable process understanding. In that regard, it is proposed that all generic process steps for each model are represented in form of a state-based control logic, which corresponds for example to the underlying control logic of the carbon fibre production line. In addition to that, the continuous behaviour and functional relations can be represented by empirically derived functions from regression analysis (as a result from the design of experiments of Module A) and/or differential equations (e.g., to account for the specific thermal process behaviour within the processes). Since all models are suggested to be built up as generic models, they can be further coupled with overall production line simulation models. This further fosters the generation of interchangeable process platforms as well as a wider transferability of results. In that sense, the approach supports a coupling of simulation models to allow for an independent development of process models, which is not exclusively linked to any specific kind of software environment. Typical software environments for process models are able to incorporate discrete and continuous behaviour such as MATLAB or Anylogic.
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Area C suggest the development of a carbon fibre multiscale production line simulation as indicated by the dashed line. Such a simulation helps to describe and forecast dynamic behaviour of energy and media flows as well as defined production performance indicators and carbon fibre characteristics. It integrates different types of carbon fibre production processes, such as oxidation, LT, HT furnace, as well as involved TBS and production planning and control (PPC) related aspects to allow for an evaluation of economic, environmental, and technological goals. This concept is based on coupled simulation models allowing the utilization of bestsuited modelling approaches and tools to represent specific production system elements such as individual processes and/or TBS technologies (e.g., for compressed air generation, cooling or (waste) heat utilization and reuse) in detail. Furthermore, the multi-scale simulation follows an agent-based structure to consider the characteristics and production requirements of individual carbon fibre types. In that context, the carbon fibres save for example section by section their individual properties. This is novel in contrast to established production line simulation approaches because it enables a flexible coupling of various individual process models (e.g., of oxidation, LT, HT furnaces or surface treatment from Module B) and provides information about process parameters and resulting carbon fibre properties. In addition, the multi-scale simulation helps to define suitable interfaces between different model types to enable the use and linkage of existing models (e.g., from material science) to new models (e.g., overall production line simulation model). This will further increase and support the re-usability of models, which reduces the effort for model creation and fosters the collaboration of different disciplines and experts in carbon fibre production such as production engineers, (final) product developers, material scientist or process/oven designers. Area D rounds off the concept by providing an interdisciplinary assessment and understanding of the model results. In that context, so-FDOOHG ³ZKDW-LI´ DQDO\VHV can be conducted to identify possible trade-offs and/or problem shifts regarding the operation of the carbon fibre line, the defined properties of the carbon fibre and the set objectives. This helps examining configurations that are particularly environmentally or economically friendly. Furthermore, key issues concerning the scale up of carbon fibre production can be evaluated from an interdisciplinary perspective comprising material scientists, production operators, product designers as well as process developers. 4. Discussion The previously introduced framework helps practitioners from the carbon fibre domain identify and assess relevant process parameters and create an understanding of the carbon fibre production process. Moreover, the framework aims to represent the whole picture and act as a guidance, when it comes to assessing individual improvement measures. This way, the effects of individual system improvements can be assessed holistically pursuant to the rest of the system and problem shifting can be prevented. While the application of the framework is still underway and empirical tests need to be
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conducted, previous work reveals first insights gained. Figure 4 lists in this context some examples of parameters that are either directly controlled as process parameters or represent process conditions that are the result of controlled process parameters and the occurring chemical reactions during the stabilisation (oxidation) process.
Fig. 4. Examples of the interplay between process parameters, process conditions and resulting fibre properties during stabilisation
The process influencing parameters are divided into predefined and adjustable parameters. Predefined parameters are derived from the oven construction and cannot be changed during operation. Although predefined parameters act as process constraints and cannot be changed during operation, they influence the arising process conditions and the TBS as well. For example, the fibre is led through different passes inside the oxidation oven (as indicated by Fig. 1). Vertical orientation of the passes requires the rollers to be placed inside the oven to avoid the escaping of contaminated hot air. This in turn requires the cooling of the rollers. In case of the horizontal orientation of the passes, the rollers can operate without cooling and can be placed outside of the oven. However, increased airflow is necessary for maintaining safe process conditions due to the poorer heat dissipating capacity [16]. Adjustable process parameters, e.g., drive speeds and air removal rate, can dynamically be changed during operation and therefore have to be adapted in accordance with the oven constraints. The air removal rate depends on the fans controlling the air in- and outflows and run-off air through seals. Careful control of the air removal rate is important for various reasons. Firstly, the concentration of noxious gases inside the oven atmosphere has to be kept below a threshold to prevent the build-up of an explosive mixture. Heating rate and temperature inherently influences chemical reactions initiating the removal of non-carbon elements from the fibre material. The elements leave the fibre in a gaseous form at characteristic temperature levels, increasing this way the concentration of (often toxic) gases in the oven atmosphere. Heating rate and airflows inside the oven are critical factors in preventing temperature run-offs due to sudden exothermic reactions. The presence of toxic gases requires in turn the operation of abatement systems for ensuring workplace safety standards for the operation staff. Secondly, some of the effluent gases slow down the conversion reactions and have to be removed to maintain continuous line speed. Line speed is controlled according to the occurring fibre shrinkage to maintain a uniform tension. Increasing tension can lead to increased fibre
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strength, but too much tension leads to surface defects, causing a drop in fibre strength. The removal of toxic gases and provision of fresh air also form a strong interrelationship with the TBS and fibre quality. The TBS can negatively affect fibre quality if temperature and humidity are not controlled uniformly over the course of the year (e.g., due to temperature differences in summer and winter) [16]. In order to reliably produce carbon fibre in a process-safe manner in the desired quality and maximize yield, the process conditions have to be controlled carefully in every process step within a safe operation window. This example illustrates the complex relationships between process parameters, final fibre properties and the interplay with the technical building services. 5. Conclusion Unfolding the interdependencies within the carbon fibre production process and desired material characteristics could help widen the application areas of carbon fibre. In this way, carbon fibre producers could respond with tailored product properties to the requirements of design engineers (e.g., highmodulus fibre, high-strength and -modulus fibre). The final carbon fibre properties emerge from the process conditions of the preceding conversion steps. However, the interrelationships between fibre properties, process conditions and correspondingly process parameters are to this day not fully understood. Therefore, this paper proposes a framework for assessing the carbon fibre production from a life cycle perspective. The discussed example underlines the various and often dynamic interconnections within the carbon fibre production system, indicating that the effects of individual improvement measures are difficult to grasp intuitively by engineers on a holistic level. To achieve the economic and environmental benign application of carbon fibre and support the scale-up of processes, there is further need for interdisciplinary research collaboration by combining methods and tools from material sciences, manufacturing equipment engineering, factory engineering and life cycle engineering. References [1] Allwood, J. M., Cullen, J. M., Carruth, M. A., & University of Cambridge. Engineering Department. (2012). Sustainable materials with both eyes open. UIT Cambridge Ltd. [2] Cullen, J. M., & Allwood, J. M. (2010). The efficient use of energy: Tracing the global flow of energy from fuel to service. Energy Policy, 38(1). [3] Luk, J. M., Kim, H. C., De Kleine, R., Wallington, T. J., & MacLean, H. L. (2017). Review of the Fuel Saving, Life Cycle GHG Emission, and Ownership Cost Impacts of Lightweighting Vehicles with Different Powertrains. Environmental Science and Technology, 51(15). [4] Liang, R., & Hota, G. (2013). Fiber-reinforced polymer (FRP) composites in environmental engineering applications. In Developments in Fiber-
Reinforced Polymer (FRP) Composites for Civil Engineering (pp. 410± 468). Elsevier. [5] Verbrugge, M., Lee, T., Krajewski, P., Sachdev, A., Bjelkengren, C., Roth, R., & Kirchain, R. (2009). Mass Decompounding and Vehicle Lightweighting. Materials Science Forum, 618±619, 411±418. [6] Duflou, J. R., Deng, Y., Van Acker, K., & Dewulf, W. (2012). Do fiberreinforced polymer composites provide environmentally benign alternatives? A life-cycle-assessment-based study. MRS Bulletin, 37(4), 374±382. [7] Wheatley, A., Warren, D., & Das, S. (2013). Development of Low-Cost Carbon Fibre for Automotive Applications. Advanced Composite Materials for Automotive Applications, 51±73. [8] Wheatley, A., Warren, D., & Das, S. (2013). Low-Cost Carbon Fibre: Applications, Performance and Cost Models. In Advanced Composite Materials for Automotive Applications (pp. 405±434). Chichester, UK: John Wiley & Sons Ltd. [9] Duflou, J. R., De Moor, J., Verpoest, I., & Dewulf, W. (2009). Environmental impact analysis of composite use in car manufacturing. CIRP Annals - Manufacturing Technology, 58(1), 9±12. [10] Huppes, G., & Ishikawa, M. (2005). Eco-efficiency and Its Terminology. Journal of Industrial Ecology, 9(4), 43±46. [11] Das, S. (2011). Life cycle assessment of carbon fiber-reinforced polymer composites. International Journal of Life Cycle Assessment, 16(3), 268± 282. [12] Egede, P. (2017). Environmental Assessment of Lightweight Electric Vehicles. Cham: Springer International Publishing. [13] Pimenta, S., & Pinho, S. T. (2011). Recycling carbon fibre reinforced polymers for structural applications: Technology review and market outlook. Waste Management, 31(2), 378±392. [14] Holmes, M. (2017). Lowering the cost of carbon fiber. Reinforced Plastics, 61(5), 279±283. [15] Huang, X. (2009). Fabrication and properties of carbon fibers. Materials, 2(4), 2369±2403. [16] Morgan, P. (2005). Carbon Fibers and Their Composites. Boca Raton: Taylor & Francis Group, LLC. [17] Park, S.-J., & Heo, G.-Y. (2015). Precursors and Manufacturing of Carbon Fibers (pp. 31±66). Dordrecht: Springer Netherlands. [18] Badii, K., Church, J. S., Golkarnarenji, G., Naebe, M., & Khayyam, H. (2016). Chemical structure based prediction of PAN and oxidized PAN fiber density through a non-linear mathematical model. Polymer Degradation and Stability, 131, 53±61. [19] Khayyam, H., Naebe, M., Bab-Hadiashar, A., Jamshidi, F., Li, Q., Atkiss, S., Buckmaster, D., & Fox, B. (2015). Stochastic optimization models for energy management in carbonization process of carbon fiber production. Applied Energy, 158, 643±655. [20] Khayyam, H., Naebe, M., Zabihi, O., Zamani, R., Atkiss, S., & Fox, B. (2015). Dynamic Prediction Models and Optimization of Polyacrylonitrile (PAN) Stabilization Processes for Production of Carbon Fiber. IEEE Transactions on Industrial Informatics, 11(4), 887±896. [21] Duflou, J. R., Sutherland, J. W., Dornfeld, D., Herrmann, C., Jeswiet, J., Kara, S., Hauschild, M., & Kellens, K. (2012). Towards energy and resource efficient manufacturing: A processes and systems approach. CIRP Annals - Manufacturing Technology, 61(2), 587±609. [22] Herrmann, C., Thiede, S., Kara, S., & Hesselbach, J. (2011). Energy oriented simulation of manufacturing systems - Concept and application. CIRP Annals - Manufacturing Technology, 60(1), 45±48. [23] Suzuki, T., & Takahashi, J. (2005). LCA of Lightweight Vehicles by Using CFRP for Mass-produced Vehicles. 15th International Conference on Composite Materials, (1), 1±5. [24] Montgomery, D.C. (2013). Design and analysis of experiments. Hoboken. John Wiley & Sons. [25] Dean, A., Morris, M., Stufken, J., & Bingham, D. (Eds.). (2015). Handbook of Design and Analysis of Experiments. Hoboken: CRC Press.
ScienceDirect Procedia CIRP 69 (2018) 43 – 48
25th CIRP Life Cycle Engineering (LCE) Conference, 30 April ± 2 May 2018, Copenhagen, Denmark
Life cycle engineering of carbon fibres for lightweight structures Antal Déra,d,*, Alexander Kaluzaa,d, Denis Kurlea, Christoph Herrmanna,d, Sami Karab, Russell Varleyc a
Chair of Sustainable Manufacturing and Life Cycle Engineering, Insitute of Machine Tools and Production Technology (IWF), Technische Universität Braunschweig, Langer Kamp 19b, 38106 Braunschweig, Germany b Sustainable Manufacturing & Life Cycle Engineering Research Group, School of Mechanical & Manufacturing Engineering, The University of New South Wales, Sydney, NSW 2052, Australia c Carbon Nexus, Institute for Frontier Materials, Deakin University, Geelong, Victoria 3216, Australia d Open Hybrid LabFactory e.V., Hermann-Münch-Straße 2, 38440 Wolfsburg, Germany * Corresponding author. Tel.: +49-531-391-65035; fax: +49-531-391-5842. E-mail address: [email protected]
Abstract Composite materials are of major importance in today´s manufacturing of lightweight structures. Due to their excellent material properties, carbon fibre reinforced plastics (CFRP) have become highly desired engineering materials for structural applications. However, energy-intensive thermal processes as well as long cycle times during manufacturing have been an obstacle towards high-volume applications. This paper presents a framework for assessing the eco-efficiency of carbon fibre production. The developed framework enables interdisciplinary collaboration between different engineering disciplines by combining methods and tools to ensure life cycle improvements in carbon fibre production. ©201 2017The The Authors. Published by Elsevier B.V. © Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license Peer-review under responsibility of the scientific committee of the 25th CIRP Life Cycle Engineering (LCE) Conference. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 25th CIRP Life Cycle Engineering (LCE) Conference
Keywords: Carbon fibre lightweighting; eco-efficiency; interdisciplinary engineering; life cycle engineering
1. Introduction Maintaining our standard of living greatly relies on transportation and industrial production that requires large amounts of energy and releases greenhouse gas emissions (GHG-emissions) into the atmosphere. Over one third of global man-made GHG emissions is generated by industry from making goods while another quarter comes from the use of transport [1, 2]. Besides alternative drivetrains like in electric mobility, the application of lightweight structures is a promising approach towards reducing the energy demand of vehicles and thus more sustainable mobility [3]. The application of fibre-reinforced plastics (FRP) plays a major role in the design and manufacturing of lightweight structures. FRP show specific advantages encompassing high strength and stiffness, low density, ease of shaping as well as a high durability in relation to other materials [4]. In high load carrying structural applications, carbon fibre-reinforced plastics
are preferable engineering materials (CFRP), both, with thermoset and thermoplastic matrix material. In automotive applications, weight reductions of up to 60 % can be realized in comparison to conventional steel designs [5]. Manufacturing of carbon fibres with state-of-the art process technology leads to high energy intensities in comparison to other lightweight engineering materials. Depending on the share of fibres in the composite (typically between 30 and 50 % by volume) including the matrix and fibre type, a cumulative energy demand that exceeds engineering steel by a factor between 5 and 10 per kilogram of material is reported [6]. Consequently, this influences the cost structure of components manufactured from CFRP, hindering the application of CFRP in price-sensitive industries [7, 8]. Towards an application in competitive product designs, burdens from manufacturing need to be compensated by benefits gained from other stages of the life cycle. Figure 1 depicts the impact of using lightweight CFRP components from an environmental perspective within
2212-8271 © 201 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 25th CIRP Life Cycle Engineering (LCE) Conference doi:10.1016/j.procir.2017.11.007
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the automotive life cycle. Lightweight automobiles have a lower energy demand and associated emissions in the use phase compared to conventionally built heavier vehicles. However, the energy intensive production of lightweight materials leads to a shift of the lifecycle hotspot towards upstream processes [9]. Consequently, the break-even point between the environmental impact and the driven distance moves further out on the mileage or time line, eventually exceeding the life of an automobile. While being an alternative to conventional materials in industries with long product life cycles and thus sufficient pay off timescales, e.g., aerospace applications, CFRP materials are currently not competitive in large-scale automotive production. To ensure the environmental advantages of lightweight structures, this break-even point has to be reached earlier in the use phase. An approach (highlighted in Figure 1) leading to this vision is to reduce the initial environmental impact during the raw material extraction and SURGXFWLRQSKDVHRIFRPSRQHQWVE\WKHIDFWRUĮ7KLVUHVXOWVLQ a shift oIĮ¶GXULQJWKHXVHphase that enables the break-even point to be reached sooner. Increasing the eco-efficiency of carbon fibre production could initiate the shift towards an earlier break-even point and be realised by reducing the energy intensity and the associated environmental impact of carbon fibre production. Eco-efficiency describes within this context the ratio between the value creation of production and its environmental impact [10]. Against this background, this paper aims to introduce a framework for assessing the eco-efficiency of carbon fibre production by combining methods and tools from different engineering disciplines.
Fig. 1. Environmental impact of using CFRP components over the automotive life cycle
2. Challenges of carbon fibre production from a LCE perspective Staying with the example of the automotive industry, the increased environmental impact in the production phase of CFRP is difficult to be compensated for within a reasonable mileage [9, 11]. In vehicle applications, the break-even point of lightweight materials is as well strongly dependent on the applied drivetrains. This is highlighted by the example of electrification. Figure 2 presents different lightweight options based on material substitutions in comparison to conventional
steel designs. It relates greenhouse gas emissions originating from the manufacturing with those resulting from the electricity mix applied for charging. The y-axis then represents the required mileage towards achieving an environmental break-even point based on state-of-the-art technologies [12]. It is to be observed that currently only markets with high shares of fossil energies seem to be advantageous for achieving the break-even point for the CFRP-lightweighting strategy of electric vehicles.
Fig. 2. Break-even of lightweight material substitutions for electric vehicles depending on GHG emissions from electricity mix applied for charging [12]
Carbon fibre-reinforced composite structures pose further challenges to the end-of-life phase of products, especially in closing the loop through re-manufacturing the recyclates into the secondary market [13]. With regard to electric vehicles the interdependencies between lightweight of the car, the battery size and the driving range are important. Reducing the manufacturing costs and environmental impact of carbon fibre production and solving the environmental obstacles in the end of life phase will undoubtedly increase demand from the automotive sector wanting to take advantage of the many advantageous properties of composite materials [7, 8, 14]. To do this, it is paramount that carbon fibre producers gain a comprehensive understanding of the interdependencies between the main production processes and quality related properties. This further includes the design and stable operation of carbon fibre production lines to improve the energy intensity and reduce the associated costs of the production processes. A first step towards improving the carbon fibre technology from a life cycle perspective is understanding its production and the underlying interdependencies between the system elements. Therefore, the next sections lead through the steps of carbon fibre production. 2.1. The carbon fibre production process Carbon fibre is a fibrous material containing at least 92 weight percent carbon [15]. It is formed in a series of processing steps from a precursor to a high-strength and highmodulus fibre. Today´s precursor materials are almost exclusively based on polyacrylonitrile (PAN) but alternatives like pitch and rayon can be employed [15]. As shown in Figure 1, carbon fibre production can be broken down into four main steps: (1) precursor fabrication, (2) stabilization (also referred to as oxidation), (3) carbonization and (4) surface treatment. Although the sequence of the processing steps
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remain the same regardless of the precursor type, the process conditions have to be adapted to the precursors individually. Precursor fabrication determines the material properties and final quality of the carbon fibre to a great extent. Moreover, it represents a major cost factor during production and has therefore been intensively investigated in recent years [7]. The following stabilisation and carbonisation steps aim at maximizing the carbon yield from the fibre. To achieve the required carbon content in the fibre, various chemical compounds (for example HCN, NH3 and CH4) are removed under carefully controlled thermal atmosphere [16]. The ongoing removal of non-carbon compounds results in a corresponding weight loss and shrinkage in fibre diameter as well as in a gradual change in fibre colour towards black. While stabilisation takes place between 200-300°C and stabilisation time is measured in hours, carbonisation requires only a few minutes at temperatures up to 3000°C [15]. The final process step, surface treatment conduces to better post-processing qualities of the carbon fibre. For example, increasing the ability to form a bond with the polymer matrix is a desired property in CFRP manufacturing [15]. In order to achieve the economic feasible and environmental benign application of carbon fibre, several challenges and unresolved obstacles need to be understood and eventually overcome. Those challenges are explained in the following bipartite structure; first addressing system relevant challenges and secondly, product, fibre and process associated interdependencies. 2.2. General challenges and drivers related to carbon fibre factories & production lines Challenges and drivers for improvements may vary among producers. While established producers will have to find ways of integrating new carbon fibre technologies into existing production lines, new market participants are more able to take advantage of future carbon fibre technologies during their own start up. Unknown future demands in combination with an increased variety of fibre types further exacerbate planning activities of production lines. Thus, it is of great interest to know the effects and impacts of a scaled-up production, particularly for the predicted growth in the automotive sector. Increased international competition among the few carbon fibre producers leads to additional cost pressures and risks for new market participants, associated with high investments in new and in existing factories. Besides economic pressures, the technological evolution and high environmental impacts of the production process itself, poses further challenges and drivers for the carbon fibre producers. As a result, addressing questions of cost and environmental impact during scale up for simultaneously improving fibre performance and quality becomes an increasing problem. This again leads to more specific questions on how to improve the manufacturing of carbon fibre (e.g., in terms of lead-time, overall equipment effectiveness, yield, scrap, personnel costs) as well as environmental perspectives (e.g., energy demand, emissions and material efficiency). Moreover, addressing individual improvement goals will inevitably affect other targets, such as
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the interconnected aforementioned indicators that create a complex production system. 2.3. Specific challenges and drivers related to products, fibres and processes Besides potentially conflicting goals and interactions between production line equipment and technical building services (TBS), the interaction between the individual production process steps and the properties of the fibres is of particular interest. Nowadays, carbon fibre production is strongly concentrated to a few producers. Furthermore, carbon fibre production is marked by either the experience of the operating personnel and/or various empirical test runs for a specific production line or individual process step [16, 17]. This circumstance impedes the production of not only reliable but also repeatable carbon fibre (e.g., of specific fibre properties) and prevents the ability to precisely control properties through an understanding of the chemistry. Consequently, it remains unknown which process parameters are necessary to achieve certain product characteristics (e.g., defined by product designers). This missing gap in knowledge creates uncertainty in the reliability and quality of the final product and decelerates scale up. In addition, currently there only exists a limited transferability of production line concepts and applications as well as a significant lack of understanding concerning the controllability of resulting fibre and therefore product properties. To meet future demands, there is a strong need for methods and tools supporting an interdisciplinary systems understanding which fosters innovations (e.g., in form of adjustable fibre properties) and an integrated evaluation of improvement measures regarding technical, economic and environmental goals. So far, only specific or isolated approaches have been developed addressing mainly the technological evaluation of either the final carbon fibres or individual process steps. Recent activities include the empirical analysis and development of prediction models for fibre properties and the oxidation process [18, 19, 20]. However, this only represents a snapshot character of the current status of the production line or specific processes, since it neglects to take further process and system interactions into account [21, 22]. The evaluation of the economic and environmental effects of carbon fibre production chiefly relies on rough estimations and assumptions (e.g., assuming an operation at full capacity, which is rarely the case) which have been addressed in several cost and life cycle assessment (LCA) studies [11, 21, 23]. However, those studies are based on static calculations and do not focus on dynamic effects, e.g., on varying process conditions required to produce different carbon fibre qualities for different application areas. A typical method to model and reflect dynamic behaviour is simulation. In manufacturing, diverse simulation approaches exist, which range from process chain over separate machines to entire factories modelling [22]. Yet, only limited effort has been made to develop simulation approaches combining multiple hierarchical scales within a carbon fibre factory and/or link them to product properties.
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Fig. 3. Conceptual framework for assessing carbon fibre production
3. Framework for assessing the eco-efficiency of carbon fibre production To tackle the outlined challenges associated with producing carbon fibres; this paper proposes a new conceptual framework to evaluate carbon fibre production. It involves empirically derived data and analytical relations to foster the interdisciplinary collaboration of relevant research fields such as material sciences, production system simulation and life cycle engineering. Therefore, the proposed concept and its methods can be perceived as a platform for experts from different research areas to share and mutually gain process knowledge and insights into the production of carbon fibres and its dynamics. Figure 3 graphically shows the abstracted and generalized framework for a carbon fibre production. The framework integrates several involved system elements and topics such as carbon fibres and their quality, individual production processes, production equipment and the involvement of TBS as well as diverse interdependencies between all those aspects. It is a key objective of the framework to suggest suitable types of modelling to examine the effects of those interdependencies for individual production line steps, such as processes (e.g., oxidation or low temperature (LT) and high temperature (HT) carbonisation furnaces) as well as changes or propagations of fibre properties along the production line. In order to examine those aspects properly, the framework is further subdivided into different research areas (A-D). Those areas differ concerning their system level ranging from single processes (A) over process fibre interactions (B) to entire production lines (C), also including an application and evaluation (D), which are described in more detail in the following. Area A has the objective to identify and quantify essential relations between carbon fibre properties pursuant to specific production process and system characteristics (e.g., temperature levels in LT and HT furnaces, overall line speed and dwell time). This lays the foundation to adjust relevant process parameters and variables in order to reach defined carbon fibre properties. To achieve that, a bipartite approach is suggested. At first, all established and acknowledged functional and analytical relations between process parameters and carbon fibre properties need to be gathered and selectively
verified. Secondly, different design of experiments have to be conducted to acquire experimental data and subsequently derive empirical knowledge about previously unknown functional relations between process parameters and carbon fibre properties. For understanding the nature of the underlying relations, for example the one factor at a time (OFAT) and the response surface methodology, based on a second-order composite experimental design, are expected to yield good results. The OFAT method varies only one parameter successively while all other parameters are kept constant. Therefore, the OFAT method represents a straightforward and simple realization. However, the OFAT method can be quite time-consuming when interactions between multiple parameters are of interest. For this reason, second-order composite experimental designs might be used to gain knowledge of parameter interaction while providing reasonable robustness against outliers [24]. If necessary, further methods of design of experiment are applicable, as well [25]. The resulting data and functional relations between process parameters and carbon fibre properties are then used as an input for the development of process specific models. Area B bases on the acquired data and functional cause-andeffect relations to model the critical production processes such as the oxidation, LT and HT furnaces to gain a fundamental and transferable process understanding. In that regard, it is proposed that all generic process steps for each model are represented in form of a state-based control logic, which corresponds for example to the underlying control logic of the carbon fibre production line. In addition to that, the continuous behaviour and functional relations can be represented by empirically derived functions from regression analysis (as a result from the design of experiments of Module A) and/or differential equations (e.g., to account for the specific thermal process behaviour within the processes). Since all models are suggested to be built up as generic models, they can be further coupled with overall production line simulation models. This further fosters the generation of interchangeable process platforms as well as a wider transferability of results. In that sense, the approach supports a coupling of simulation models to allow for an independent development of process models, which is not exclusively linked to any specific kind of software environment. Typical software environments for process models are able to incorporate discrete and continuous behaviour such as MATLAB or Anylogic.
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Area C suggest the development of a carbon fibre multiscale production line simulation as indicated by the dashed line. Such a simulation helps to describe and forecast dynamic behaviour of energy and media flows as well as defined production performance indicators and carbon fibre characteristics. It integrates different types of carbon fibre production processes, such as oxidation, LT, HT furnace, as well as involved TBS and production planning and control (PPC) related aspects to allow for an evaluation of economic, environmental, and technological goals. This concept is based on coupled simulation models allowing the utilization of bestsuited modelling approaches and tools to represent specific production system elements such as individual processes and/or TBS technologies (e.g., for compressed air generation, cooling or (waste) heat utilization and reuse) in detail. Furthermore, the multi-scale simulation follows an agent-based structure to consider the characteristics and production requirements of individual carbon fibre types. In that context, the carbon fibres save for example section by section their individual properties. This is novel in contrast to established production line simulation approaches because it enables a flexible coupling of various individual process models (e.g., of oxidation, LT, HT furnaces or surface treatment from Module B) and provides information about process parameters and resulting carbon fibre properties. In addition, the multi-scale simulation helps to define suitable interfaces between different model types to enable the use and linkage of existing models (e.g., from material science) to new models (e.g., overall production line simulation model). This will further increase and support the re-usability of models, which reduces the effort for model creation and fosters the collaboration of different disciplines and experts in carbon fibre production such as production engineers, (final) product developers, material scientist or process/oven designers. Area D rounds off the concept by providing an interdisciplinary assessment and understanding of the model results. In that context, so-FDOOHG ³ZKDW-LI´ DQDO\VHV can be conducted to identify possible trade-offs and/or problem shifts regarding the operation of the carbon fibre line, the defined properties of the carbon fibre and the set objectives. This helps examining configurations that are particularly environmentally or economically friendly. Furthermore, key issues concerning the scale up of carbon fibre production can be evaluated from an interdisciplinary perspective comprising material scientists, production operators, product designers as well as process developers. 4. Discussion The previously introduced framework helps practitioners from the carbon fibre domain identify and assess relevant process parameters and create an understanding of the carbon fibre production process. Moreover, the framework aims to represent the whole picture and act as a guidance, when it comes to assessing individual improvement measures. This way, the effects of individual system improvements can be assessed holistically pursuant to the rest of the system and problem shifting can be prevented. While the application of the framework is still underway and empirical tests need to be
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conducted, previous work reveals first insights gained. Figure 4 lists in this context some examples of parameters that are either directly controlled as process parameters or represent process conditions that are the result of controlled process parameters and the occurring chemical reactions during the stabilisation (oxidation) process.
Fig. 4. Examples of the interplay between process parameters, process conditions and resulting fibre properties during stabilisation
The process influencing parameters are divided into predefined and adjustable parameters. Predefined parameters are derived from the oven construction and cannot be changed during operation. Although predefined parameters act as process constraints and cannot be changed during operation, they influence the arising process conditions and the TBS as well. For example, the fibre is led through different passes inside the oxidation oven (as indicated by Fig. 1). Vertical orientation of the passes requires the rollers to be placed inside the oven to avoid the escaping of contaminated hot air. This in turn requires the cooling of the rollers. In case of the horizontal orientation of the passes, the rollers can operate without cooling and can be placed outside of the oven. However, increased airflow is necessary for maintaining safe process conditions due to the poorer heat dissipating capacity [16]. Adjustable process parameters, e.g., drive speeds and air removal rate, can dynamically be changed during operation and therefore have to be adapted in accordance with the oven constraints. The air removal rate depends on the fans controlling the air in- and outflows and run-off air through seals. Careful control of the air removal rate is important for various reasons. Firstly, the concentration of noxious gases inside the oven atmosphere has to be kept below a threshold to prevent the build-up of an explosive mixture. Heating rate and temperature inherently influences chemical reactions initiating the removal of non-carbon elements from the fibre material. The elements leave the fibre in a gaseous form at characteristic temperature levels, increasing this way the concentration of (often toxic) gases in the oven atmosphere. Heating rate and airflows inside the oven are critical factors in preventing temperature run-offs due to sudden exothermic reactions. The presence of toxic gases requires in turn the operation of abatement systems for ensuring workplace safety standards for the operation staff. Secondly, some of the effluent gases slow down the conversion reactions and have to be removed to maintain continuous line speed. Line speed is controlled according to the occurring fibre shrinkage to maintain a uniform tension. Increasing tension can lead to increased fibre
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strength, but too much tension leads to surface defects, causing a drop in fibre strength. The removal of toxic gases and provision of fresh air also form a strong interrelationship with the TBS and fibre quality. The TBS can negatively affect fibre quality if temperature and humidity are not controlled uniformly over the course of the year (e.g., due to temperature differences in summer and winter) [16]. In order to reliably produce carbon fibre in a process-safe manner in the desired quality and maximize yield, the process conditions have to be controlled carefully in every process step within a safe operation window. This example illustrates the complex relationships between process parameters, final fibre properties and the interplay with the technical building services. 5. Conclusion Unfolding the interdependencies within the carbon fibre production process and desired material characteristics could help widen the application areas of carbon fibre. In this way, carbon fibre producers could respond with tailored product properties to the requirements of design engineers (e.g., highmodulus fibre, high-strength and -modulus fibre). The final carbon fibre properties emerge from the process conditions of the preceding conversion steps. However, the interrelationships between fibre properties, process conditions and correspondingly process parameters are to this day not fully understood. Therefore, this paper proposes a framework for assessing the carbon fibre production from a life cycle perspective. The discussed example underlines the various and often dynamic interconnections within the carbon fibre production system, indicating that the effects of individual improvement measures are difficult to grasp intuitively by engineers on a holistic level. To achieve the economic and environmental benign application of carbon fibre and support the scale-up of processes, there is further need for interdisciplinary research collaboration by combining methods and tools from material sciences, manufacturing equipment engineering, factory engineering and life cycle engineering. References [1] Allwood, J. M., Cullen, J. M., Carruth, M. A., & University of Cambridge. Engineering Department. (2012). Sustainable materials with both eyes open. UIT Cambridge Ltd. [2] Cullen, J. M., & Allwood, J. M. (2010). The efficient use of energy: Tracing the global flow of energy from fuel to service. Energy Policy, 38(1). [3] Luk, J. M., Kim, H. C., De Kleine, R., Wallington, T. J., & MacLean, H. L. (2017). Review of the Fuel Saving, Life Cycle GHG Emission, and Ownership Cost Impacts of Lightweighting Vehicles with Different Powertrains. Environmental Science and Technology, 51(15). [4] Liang, R., & Hota, G. (2013). Fiber-reinforced polymer (FRP) composites in environmental engineering applications. In Developments in Fiber-
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